1. Field of Endeavor
The invention relates to a method and also a device for combusting gaseous fuel, which contains hydrogen or consists of hydrogen, with a burner which provides a swirl generator into which liquid fuel, for example mineral oil, is feedable centrally along a burner axis, forming a liquid fuel column which is conically formed, and which is enveloped by, and mixed through with, a rotating combustion air flow which flows tangentially into the swirl generator. Furthermore, devices for feeding gaseous fuel, for example natural gas, are provided in the combustion air flow which enters the swirl generator through tangential air inlet slots.
2. Brief Description of the Related Art
Motivated by the almost worldwide endeavor with regard to the reduction of emission of greenhouse gases into the atmosphere, not least of all established in the so-called Kyoto Protocol, the emission of greenhouse gases which is to be expected in the year 2010 is to be reduced to the same level as in the year 1990. To implement this plan, it requires greater efforts, especially to reduce the contribution to anthropogenic-related CO2 releases. Approximately a third of the CO2 which is released by people into the atmosphere is directed back for energy generation, in which mostly fossil fuels are combusted in power generating plants for the generation of electricity. Especially by the use of modern technologies and also by additional political parameters, a significant potential for economy for avoiding a further increasing of CO2 emission can be seen by the energy-generating sector.
An as known per se and technically controllable possibility to reduce the CO2 emission in combustion power plants consists in the extraction of carbon from the fuels, which are obtained for combustion, before introducing the fuel into the combustion chamber. This requires corresponding pretreatments of fuel, as, for example, the partial oxidation of the fuel with oxygen and/or a pretreatment of the fuel with water vapor. Such pretreated fuels mostly have a large portion of H2 and CO, and depending upon mixing ratios, have calorific values which, as a rule, are below those of normal natural gas. In dependence upon their calorific value, gases which are synthetically produced in such a way are referred to as Mbtu or Lbtu gases, which are not simply suitable for use in conventional burners which are designed for the combustion of normal gases such as natural gas, as they can be gathered, for example, from EP 0 321 809 B1, EP 0 780 629 A2, WO 93/17279 and also EP 1 070 915 A1. In all of the preceding documents, burners of the fuel premixing type are described, in which a swirled flow of combustion air and admixed fuel, which conically expands in the flow direction, is generated in each case, which swirled flow becomes unstable in the flow direction by means of the increasing swirl after exit from the burner, as far as possible after achieving a homogenous air-fuel mixture, and changes into an annular swirled flow with backflow in the core.
Depending upon the burner concept and also in dependence upon the burner capacity, the swirled flow of liquid and/or gaseous fuel, which is formed inside the premix burner, is fed for forming a fuel-air mixture which is as homogenous as possible. However, as previously mentioned, it is necessary to use synthetically prepared gaseous fuels alternatively to or in combination with the combustion of conventional fuel types for the purpose of a reduced emission of pollutants, especially the emission of CO2, so special requirements arise for the constructional design of conventional premix burner systems. Consequently, for feed into burner systems, synthesis gases require a multiple volumetric flow of fuel compared with comparable burners which are operated with natural gas, so that appreciably different flow impulse conditions are created. On account of the high portion of hydrogen in the synthesis gas, and the low ignition temperature and high flame velocity of the hydrogen which are connected with it, there is a high reaction tendency of the fuel, which leads to an increased risk of backflash. In order to avoid this, it is necessary to reduce as far as possible the average retention time of ignitable fuel-air mixture inside the burner.
A method and also a burner for combustion of gaseous fuel, liquid fuel, and also medium-calorific or low-calorific fuel, is described in EP 0 908 671 B1. In this case, a double-cone burner with a downstream mixing path according to EP 0 780 629 A2 is used, in the swirl shells of which, which define the swirl chamber, feed pipes for axial and/or coaxial injection of medium-calorific or low-calorific fuel into the inside of the swirl generator are provided. A schematic assembly of such a premix burner arrangement is shown in FIGS. 2 and 3 herein. FIG. 2 shows a longitudinal section, FIG. 3 shows a cross section through the premix burner arrangement which provides a conically widening swirl generator 1 which is defined by swirl shells 2. Devices for feeding fuel are provided axially and also coaxially around the center axis A of the swirl generator 1. Therefore, liquid fuel BL reaches the swirl chamber by means of an injection nozzle 3 which is positioned along the burner axis A at the location of the smallest inside diameter of the swirl generator 1. Gaseous fuel BG, preferably in the form of natural gas, is admixed with the combustion air along tangential air inlet slots 4 through which combustion air L enters the swirl chamber with a tangential flow direction. Injection devices 5 are additionally provided, which are coaxially arranged around the burner axis A and serve for the additional feed of medium-calorific fuel BM.
The fuel-air mixture which is formed inside the swirl generator 1 reaches a mixer tube 8, in the form of a swirled flow, through a transition piece 6 which provides the flow medium 7 which stabilizes the swirled flow, in which mixer tube a complete homogenous mixing through is carried out of the fuel-air mixture which is formed, before the ignitable fuel-air mixture is ignited inside a combustion chamber (not shown) which is connected downstream to the mixer tube 8. FIG. 3 shows a cross section through the swirl generator 1 in the region of the injection devices 5 which penetrate the swirl shells 2. The air inlet slots 4, through which air L penetrates into the inside of the swirl generator 1, are better visible in the cross sectional view. Gaseous fuel BG, via corresponding feed pipes, is admixed together with the combustion air L at the location of the air inlet slots 4. An injection nozzle for the delivery of liquid fuel into the inside of the swirl generator 1 is provided centrally to the burner axis A.
The combustion of medium-calorific fuels, the calorific values of which are typically between 5 MJ/kg and 15 MJ/kg, is indeed possible with the previously described burner concept in hybrid operating mode alone or in combination with the combustion of liquid fuel and natural gas, yet extensive combustion trials have revealed that, while endeavoring to use fuels which are as carbon-free as possible and which in addition have a hydrogen portion which is as large as possible and which preferably consist completely of hydrogen, the use of the previously described premix burner is not suitable. Since fuels which are rich in hydrogen, with a hydrogen portion of more than 50 percent, have such a high reactivity and also a very much higher flame velocity, which typically is twice as much as that of flames which are operated with medium-calorific synthesis gases, and, furthermore, have a very much lower volume of specific heat calorific value (MJ/m3), there is a need for a very much larger quantity of hydrogen which has to be fed to the burner for achieving a desired combustion heat. Especially when using fuel which exclusively consists of hydrogen, high-pressure trials on a generic type premix burner for operating a gas turbine plant, the operation of which requires high firing temperatures, showed that ignition phenomena already occur in the swirl chamber or along the mixing path of the burner, as the case may be, which are attributed to an inadequate mixing of the hydrogen which is fed axialwards into the burner with large volumetric flow. Even in cases in which no backflash phenomena occur, inadequate mixing of hydrogen and combustion air provides a diffusion-like combustion, which ultimately leads to increased emissions of nitrogen oxide.